(a) Field of the Invention
The present invention relates to a fan speed control circuit and, more particularly, to a fan speed control circuit capable of reducing vibrations and noises in a fan motor by using a phase control method.
(b) Description of the Related Art
Among conventional fan speed control technologies, pulse width modulation (PWM) control is the most common method used to control the speed of direct current (DC) motors.
Referring to FIG. 1, in a conventional PWM control method, an external PWM signal 102 is used to drive a bipolar transistor 104 and a driver IC 106 such that the terminal voltage of a stator coil 108 is controlled. Also, a hall element 110 whose output voltage is directly proportional to the magnetic field strength is used to sense the magnetic polarity of the rotor in a fan motor.
Referring to FIG. 2, the fan motor is “on-time” as the pulse level of the incoming PWM signal 102 is high (also called the duty cycle), while the fan motor is “off-time” as the pulse level is low. The sum of these two times is one period. Therefore, one can, for instance, decrease the on-time or increase the off-time of the fan motor in one period to lower the motor speed.
In a brushless DC motor, cogging torque is produced by the magnetic attraction between the rotor mounted on permanent magnets and the stator, and it is an undesired effect that makes precise positioning of the rotor impossible because the rotor tends to lock onto the position where it is aligned with the stator poles. Further, when the rotor rotates, the magnetic flux variation causes back electromotive force (back emf) effect, and the back emf effect in turn enhances the cogging torque to result in high values of output ripple, vibrations, and noises in the fan motor.
FIG. 3A shows a conventional motor driving system 1 mainly consisting of a driver 210 and a coil 211 wherein the coil 211 is electrically connected with the driver 210, and the driver 210 is electrically connected with a power generator 212 and receives a power Vcc from the power generator. The power Vcc provides the driver 210 with the necessary working voltage and the driver 210 outputs a driving current to the coil 211 based on the working voltage.
FIG. 3B shows another conventional motor driving system 1 differing from the aforementioned motor driving system 1 in that the driver 210 is electrically connected with a plurality of coils 211 (only two coils are shown in FIG. 3B).
In these two kinds of motor driving systems 1 mentioned above, the motor speed is controlled by varying the power Vcc. However, such kind of speed control technique is limited by the working voltage of the driver 210 thereby resulting in a narrow speed-control range, and therefore cannot satisfy the industry requirement, gradually.
FIG. 4A and FIG. 4B show a conventional motor driving system 2 wherein the driver 210 and the coil 211 of FIG. 3A and FIG. 3B are applied as a fan motor 220. The fan motor 220 of FIG. 4A has one end electrically connected with a power generator 223 for receiving a power Vcc from the power generator and the other end electrically connected with a collector C of a transistor 221. The transistor 221 has an emitter E electrically connected with a ground terminal and a base B electrically connected with a PWM signal source 222. The motor driving system of FIG. 4B differs from that of FIG. 4A mainly in that the fan motor 220 has one end electrically connected with a ground terminal and the other end electrically connected with the collector of the transistor 221, and the emitter E of the transistor 220 is electrically connected with the power generator 223. In the motor driving systems 2 shown in FIG. 4A and FIG. 4B, the base B of the transistor 220 receives a PWM signal (Spwm) from the PWM source 222, and the transistor 221 is turned on or off according to the PWM signal thereby controlling the speed of the fan motor 220. In this kind of control method, the speed modulation of the fan motor 220 is achieved by performing the ON/OFF action. However, the driver (not shown) in the fan motor 220 also performs switching action while the ON/OFF action is performed, thereby producing on/off switching noise which in turn causes electric noise. Therefore, it is not a good design and needs to be improved.